Electromagnetic surveying for hydrocarbon reservoirs

ABSTRACT

An electromagnetic survey method for surveying an area previously identified as potentially containing a subsea hydrocarbon reservoir, comprising obtaining first and second survey data sets with an electromagnetic source aligned end-on and broadside relative to the same or different receivers. The invention also relates to planning a survey using this method, and to analysis of survey data taken in combination allow the galvanic contribution to the signals collected at the receiver to be contrasted with the inductive effects, and the effects of signal attenuation, which are highly dependent on local properties of the rock formation, overlying water and air at the survey area. This is very important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes of structure.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/497,807, filed Jun. 4, 2004, issued as U.S. Pat. No. 7,126,338, whichin turn is a is a national phase of International Application No.PCT/GB02/05355 filed Nov. 28, 2002 and published in the Englishlanguage, the disclosures of which are incorporated herein in theirentirety by reference.

BACKGROUND OF THE INVENTION

The invention relates to seafloor electromagnetic surveying for oil andother hydrocarbon reserves.

Determining the response of the sub-surface strata within the earth'scrust to electromagnetic fields is a valuable tool in the field ofgeophysical research. The geological processes occurring in thermally,hydrothermally or magmatically active regions can be studied. Inaddition, electromagnetic sounding techniques can provide valuableinsights into the nature, and particularly the likely hydrocarboncontent, of subterranean reservoirs in the context of subterranean oilexploration and surveying.

Seismic techniques are often used during oil-exploration expeditions toidentify the existence, location and extent of reservoirs insubterranean rock strata.

Whilst seismic surveying is able to identify such structures, thetechnique is often unable to distinguish between the different possiblecompositions of pore fluids within them, especially for pore fluidswhich have similar mechanical properties. In the field of oilexploration, it is necessary to determine whether a previouslyidentified reservoir contains oil or just aqueous pore fluids. To dothis, an exploratory well is drilled to determine the contents of thereservoir. However, this is an expensive process, and one which providesno guarantee of reward.

Whilst oil-filled and water-filled reservoirs are mechanically similar,they do possess significantly different electrical properties and theseprovide for the possibility of electromagnetic based discriminationtesting. A known technique for electromagnetic probing of subterraneanrock strata is the passive magneto-telluric (MT) method. The signalmeasured by a surface-based electromagnetic detector in response toelectromagnetic (EM) fields generated naturally, such as within theearth's upper atmosphere, can provide details about the surroundingsubterranean rock strata.

However, for deep-sea surveys, all but those MT signals with periodscorresponding to several cycles per hour are screened from the seafloorby the highly conductive seawater. Whilst the long wavelength signalswhich do penetrate to the seafloor can be used for large scale underseaprobing, they do not provide sufficient spatial resolution to examinethe electrical properties of the typically relatively small scalesubterranean reservoirs. Moreover, since MT surveying relies primarilyon horizontally polarised EM fields, it is intrinsically insensitive tothin resistive layers.

Nonetheless, measurements of electrical resistivity beneath the seafloorhave traditionally played a crucial role in hydrocarbon exploration andreservoir assessment and development. In industry, sub-seafloorresistivity data have generally been obtained almost exclusively bywire-line logging of wells. There are, though, clear advantages todeveloping non-invasive geophysical methods capable of providing suchinformation. Although inevitably such methods would be unable to providecomparable vertical resolution to wireline logging, the vast saving interms of avoiding the costs of drilling test wells into structures thatdo not contain economically recoverable amounts of hydrocarbon wouldrepresent a major economic advantage.

In research fields that are not of commercial interest, geophysicalmethods for mapping sub-seafloor resistivity variations by various formsof electromagnetic surveying have been under development for many years(e. g. Sinha et al., 1990; Evans et al., 1994). WO 00/13046 and WO00/57555 make proposals for finding hydrocarbon reservoirs using suchelectromagnetic surveying.

SUMMARY OF THE INVENTION

The invention discloses a new approach for electromagnetic surveying tolocate hydrocarbon layers. New source-detector geometries are used basedupon an electromagnetic source.

According to a first aspect of the invention there is provided anelectromagnetic survey method for surveying an area previouslyidentified as potentially containing a subsea hydrocarbon reservoir,comprising: obtaining first and second survey data sets with anelectromagnetic source aligned end-on relative to a first detector andaligned broadside relative to a second detector.

The terms source and detector are used interchangeably with transmitterand receiver respectively throughout this document.

The survey data from end-on and broadside alignments taken incombination allow the difference between galvanic and inductivecontributions to the signals collected at the detector to be determined.Collecting survey data highlighting only the galvanic contribution in anend-on geometry is not reliable. As is demonstrated in the examplesgiven below, it is generally impossible to differentiate between a rockformation containing a hydrocarbon reservoir and one which does notcontain a hydrocarbon reservoir by studying the end-on survey dataalone. The previously proposed electromagnetic survey methods forfinding hydrocarbon reservoirs are thus believed to be at best highlyunreliable.

The end-on survey data are sensitive to the presence of resistivehydrocarbon layers (exploiting largely galvanic effects and verticalcomponents of induced current flow). By contrast, the broadside surveydata are sensitive to the larger scale structure, but relativelyinsensitive to resistive hydrocarbon layers (exploiting the dominantlyinductive effects). The reason why collection of survey data from thebroadside geometry is essential for reliable electromagnetic surveyingis that many features other than hydrocarbon reservoirs can affect theresistivity beneath the seafloor and the results of a survey. Forexample, resistivity often increases steadily with depth in submarinesedimentary basins, due to the progressive expulsion of pore fluids byrising overburden pressure. Such a resistivity profile will produceeffectively the same response in the end-on survey data, as wouldpresence of a hydrocarbon layer.

Comparative examples given below demonstrate this effect.

In the preferred implementation of the survey of the first aspect of theinvention, the end-on and broadside alignments correspond to anarrangement of the electromagnetic source and the first and seconddetectors in which a right angle is formed between a first line leadingfrom the first detector to the source and a second line leading from thesecond detector to the source, and wherein the source has its dipoleaxis aligned along the first line. However, in practice, an onlyapproximate satisfaction of this condition will not greatly reduce thequality of the collected survey data. In any case, this ideal conditionwill not be satisfied exactly in practice, since the source is typicallymoved during surveying, being in the form of an antenna towed by anunmanned submarine craft. It would also be possible to obtain useabledata if the above-mentioned right angle was changed to an angle awayfrom 90 degrees, for example anywhere from 45-135 degrees may besatisfactory. How much the quality of the data deteriorates as thesurvey geometry moves away from right angles has not been studied,although this would be straightforward to do using the modellingtechniques described herein.

With the first aspect of the invention, the first and second survey datasets are preferably obtained concurrently. This can be achieved during asingle tow of the electromagnetic source. The data collected by thedetectors can then be time-synchronised to the same absolute clock.

According to a second aspect of the invention there is provided anelectromagnetic survey method for surveying an area previouslyidentified as potentially containing a subsea hydrocarbon reservoir,comprising: obtaining first and second survey data sets with anelectromagnetic source aligned end-on and broadside respectivelyrelative to a first detector.

The method of the second aspect of the invention thus differs from thatof the first aspect in that a single detector can be used to collectboth the end-on and broadside survey data. This can be done by towingthe source twice, once in a direction along the line connecting thesource to the detector, and again in a direction transverse thereto.Namely, the first and second survey data sets can be obtainedconsecutively. The relative alignment between the source and detectorwhen the end-on and broadside survey data are collected can be varied inthe same way as discussed above in relation to the first aspect of theinvention. In other words, it is best if the dipole is aligned along aline connecting the source and detector when the end-on survey data iscollected, and aligned perpendicular to that line when the broadsidesurvey data is collected. However, deviations from that condition willoccur and can be tolerated.

It will also be understood that the first survey data set can beobtained before or after second data survey set using the sameelectromagnetic source.

In principle, the first and second survey data sets could be obtainedwith separate sources carried by different transmitters, in which casethe data sets could be obtained concurrently. However, in practice, itis likely that only a single source will be deployed and the first andsecond data sets will thus be obtained one after the other.

According to a third aspect of the invention there is provided a methodof analysing results from an electromagnetic survey of an areapotentially containing a subsea hydrocarbon reservoir, comprising:providing first and second survey data sets obtained from anelectromagnetic source aligned respectively end-on and broadsiderelative to a detector; and combining the first and second survey datasets to obtain a results data set that represents a difference betweenthe end-on and broadside alignments.

The method can be greatly improved by normalising each of the first andsecond survey data sets relative to respective first and secondnormalisation data sets or functions specific to the end-on andbroadside alignments respectively, prior to the combining.

The first and second normalisation data sets or functions can becalculated from a rock formation model, or from the first and secondsurvey data sets.

In a preferred embodiment, the first and second data sets each compriseradial and azimuthal components of electric field or magnetic fieldmeasured at the detector, and the method further comprises: transformingthe radial and azimuthal components into at least one polarisationellipse parameter, prior to the combining. The polarisation ellipseparameter (s) can be the amplitude and/or phase of the component of theelectric field or magnetic field aligned along a major axis of theellipse.

The method may advantageously further comprise: visually representingthe results data set in a plot of at least two dimensions correspondingto the survey area.

The visual representation can be a two-dimensional (2D) plot in planview, or a three-dimensional (3D) plot including depth, e. g. aperspective view of the survey area.

The plot may include markings of areas of equal or similarelectromagnetic field strength. These markings could be contour lines,or colour or grey scale gradations with one colour or grey tone beingused for a range of electromagnetic field strength values. Stepwisecolour gradation is used in the preferred embodiment, with each colourrepresenting a defined range of data values.

When the plot is of normalised survey data it is helpful if the plotalso includes lines of equal absolute electromagnetic field strength,showing how the signal strength has decayed as one moves away from thesource. The lines, which may appear as contours, can be labelled witheither a relative or absolute decay value.

According to a fourth aspect of the invention, there is provided amethod of planning an electromagnetic survey of an area identified aspotentially containing a subsea hydrocarbon reservoir, comprising:creating a model of the area to be surveyed, including a rock formationcontaining a hydrocarbon reservoir and a body of water above the rockformation; setting values for water depth, depth below the seafloor ofthe hydrocarbon reservoir, and resistivity structure of the rockformation; performing a simulation of an electromagnetic survey in themodel of the survey area by calculating first and second survey datasets for an electromagnetic source aligned end-on and broadside relativeto a detector; and combining the first and second survey data sets toobtain a results data set that represents a difference between theend-on and broadside alignments.

In typical use, the simulation will be repeated for a number of sourcefrequencies and/or source-to-detector distances, and/or other parameterswhich can be varied during a survey. This iterative procedure can beused in order to select optimum surveying conditions in terms of sourcefrequency and source-to-detector distance for probing the hydrocarbonreservoir. By optimum, it is not necessarily meant that the best surveyconditions are found, but only that a set of survey conditions is foundwhich will provide strong, unmistakable indications in the case thatthere is a hydrocarbon reservoir at the survey site. The iterativeprocedure may be purely under manual control. However, preferably, thesimulator can allow the user the option of automatically optimising thesurvey conditions. The user can then switch between manual and automatediteration as desired.

The model should preferably include a body of air above the body ofwater, so that the simulation can take account of signal propagationpaths including the body of air when calculating the first and secondsurvey data sets. The propagation path through the air (the ‘air wave’)will in fact dominate for shallower water and longer distances betweensource and detector (s), as will be apparent from the examples describedfurther below.

For deep water and shorter detector-source distances the effect is lessimportant and may be omitted from the model.

The method preferably further comprises: normalising each of the firstand second survey data sets relative to respective first and secondnormalisation data sets or functions specific to the end-on andbroadside alignments respectively, prior to the combining. The first andsecond normalisation data sets or functions can be calculated from arock formation model, for example.

The method preferably further comprises visually representing theresults data set in a plot of at least two dimensions corresponding tothe survey area. The visual representation can be a 2D plot in planview, or a 3D plot including depth, e. g. a perspective view of thesurvey area. The other comments made above in relation to the plots ofthe third aspect of the invention also apply to the fourth aspect of theinvention.

Another aspect of the invention relates to a computer program productbearing machine readable instructions for implementing the method ofanalysing results from an electromagnetic survey according to the thirdaspect of the invention.

Another aspect of the invention relates to a computer apparatus loadedwith machine readable instructions for implementing the method ofanalysing results from an electromagnetic survey according to the thirdaspect of the invention.

Another aspect of the invention relates to a computer program productbearing machine readable instructions for implementing the method ofplanning an electromagnetic survey according to the fourth aspect of theinvention.

Another aspect of the invention relates to a computer apparatus loadedwith machine readable instructions for implementing the method ofplanning an electromagnetic survey according to the fourth aspect of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings.

FIG. 1 shows a schematic representation of an exploratory EM soundingsurvey.

FIG. 2A is a graph showing the modelled electric field strength as afunction of distance from, and along the axis of, the EM source. This isshown for several frequency components. The-modelled sea depth is 750 m.

FIG. 2B is a graph showing the modelled electric field strength as afunction of distance from, and along the axis of, the EM source. This isshown for several frequency components. The modelled sea depth is 900 m.

FIG. 2C is a graph showing the modelled electric field strength as afunction of distance from, and along the axis of, the EM source. This isshown for several frequency components. The modelled sea depth is 1300m.

FIG. 3 is a schematic plan view defining a survey geometry coordinatesystem.

FIG. 4A shows a schematic section view of a model subterranean strataconfiguration containing a hydrocarbon reservoir.

FIG. 4B shows a schematic section view of a model subterranean strataconfiguration containing layers of increasing resistivity.

FIG. 4C is a graph showing the electric field strength (E₀, E₉₀) as afunction of distance from the source for two different detectorpositions for the subterranean strata shown in FIG. 4A. Also shown arethe normalising signals (N₀, N₉₀) expected from a uniform subterraneanstrata configuration.

FIG. 4D is a graph showing the electric field strength (N₀, N₉₀) as afunction of distance from the source for two different detectorpositions for the subterranean strata shown in FIG. 4B. Also shown arethe normalising signals (N₀, N₉₀) expected from a uniform subterraneanstrata configuration.

FIG. 4E is a graph showing the electric field strengths (N₀, N₉₀) shownin FIG. 4C normalised by the normalising field strengths (N₀, N₉₀) shownin the same.

FIG. 4F is a graph showing the electric field strengths (N₀, N₉₀) shownin FIG. 4D normalised by the normalising field strengths (N₀, N₉₀) shownin the same.

FIG. 5 is a schematic plan view showing an arrangement of sixteendetectors on a section of sea floor above a subterranean reservoir, aparticular source transmitter tow path is also shown.

FIG. 6A shows a schematic section view of a model subterranean strataconfiguration containing a hydrocarbon reservoir.

FIG. 6B is a plan view representing the normalised electric fieldstrength as a function of location for the model subterranean strataconfiguration shown in FIG. 6A for a first transmitter position.

FIG. 6C is a plan view representing the normalised electric fieldstrength as a function of location for the model subterranean strataconfiguration shown in FIG. 6A for a second transmitter position.

DETAILED DESCRIPTION

FIG. 1 of the accompanying drawing shows schematically a surface vessel14 undertaking EM sounding of subterranean rock strata 8 within which ahydrocarbon reservoir 12 is located. The surface vessel 14 floats on thesurface 2 of the sea 4. A deep-towed vehicle 18 is attached to thesurface vessel 14 by an umbilical cable 16 which provides an electrical,optical and mechanical connection between the deep-towed vehicle 18 andthe surface vessel 14. The deep-towed vehicle 18 is towed by the surfacevessel 14 such that it remains consistently close to the seafloor 6.This is facilitated by an echo-location package 20 which relaysinformation about the height of the deep-towed vehicle 18 above theseafloor 6 to the surface vessel 14. The deep-towed vehicle 18 receiveselectrical power from the ship's on-board power supply via the umbilicalcable 16.

A cycloconverter unit 30 generates the chosen waveform to be supplied toan EM source in the form of a transmitting antenna 22 which is towed bythe deep-towed vehicle 18. The transmitting antenna 22 broadcasts the EMsignal into the sea 4, and this results in a component passing throughthe rock strata 8. One or more remote instrument packages 26 record thesignal received by receiving antennae 24 in response to the transmittedEM signal. If the separation of the transmitting antenna 22 and thereceiving antenna 24 is greater than a few hundred meters, the highlyconductive seawater strongly attenuates the direct signal between them.The components of the EM signal that have travelled through the rockstrata 8 and the reservoir 12 dominate the received signal and provideinformation about the electrical properties of these regions. At the endof the sounding experiment, a remotely operable release system allowsthe instrument package 26 to be detached from a ballast weight (notshown) so that an in-built flotation device 28 can carry the instrumentpackage 26 to the surface 2 for recovery and retrieval of data forinversion analysis.

The transmitting antenna 22 emits signals that propagate outwards bothinto the overlying water colunm 4 and downwards into the seafloor 6 andthe underlying strata 8,12. In both cases, at practical frequencies forthis method and given the typical resistivity of the media 4, 8, 12,propagation occurs by diffusion of electromagnetic fields. The rate ofdecay in amplitude and the phase shift of the signal are controlled bothby geometric spreading and by skin depth effects. Because in general theunderlying strata 8,12 are more resistive than seawater 4, skin depthsin the underlying strata 8,12 are longer. As a result, electric fieldsmeasured at the seafloor 6 by a receiving antenna 24 at a suitablehorizontal range are dominated by the components of the source fieldswhich have propagated downwards through the seafloor 6, along within theunderlying strata 8, 12, and back up to the receiving antenna 24. Boththe amplitude and the phase of the received signal depend on theresistivity structure of the underlying strata 8, 12—and so, inprincipal, a survey consisting of many transmitter (source) and receiver(detector) locations can provide a multi-dimensional image, bygeophysical inversion, of sub-seafloor resistivity.

The technique described here exploits the large resistivity contrastthat exists between a hydrocarbon reservoir (typically tens of Ω m orhigher) and the over-and under-lying sediments (typically ˜2 Ωm orless). Such a contrast has a detectable influence on controlled sourceelectromagnetic data collected at the seafloor 6 above the reservoir 12.The effect of the reservoir is most detectable in controlled sourceelectromagnetic data at an appropriate frequency, and if the horizontalrange from source 22 to receiver 24 is of the order of 2 to 5 times thedepth of burial of the reservoir 12 in typical situations.

This following text describes specific geometric and data reductionapproaches that allow the effect of a hydrocarbon reservoir on theoutcome of a controlled source electromagnetic survey to be detected andanalysed in practice.

Use of a Mobile Source and Multiple Fixed Receivers

In order to achieve a satisfactory survey outcome, it is essential tomake controlled source electromagnetic measurements over a broad rangeof survey geometries —in other words, many transmitting locations andmany receiving locations. The transmitter, i. e. the transmittingantenna, requires significant power to drive it, of the order tens ofkilowatts or greater for signals detectable at ranges of severalkilometers. It must therefore be connected by an umbilical cable 16 tothe survey vessel 14 and this makes it relatively straightforward tomake the transmitter mobile. It can then be towed in an appropriatesurvey pattern from the surface survey vessel 14. Since in manysituations surveys of this kind are liable to take place over areas ofthe seafloor 6 where sensitive engineering installations exist or areplanned, there are significant advantages to using a transmitter whichdoes not physically come into contact with the seafloor 6. Provided thatthe separation between the transmitting antenna 22 and the seafloor 6 issmall compared to a skin depth of the investigating field in seawater,the survey can still be completed satisfactorily. As a result, the mostappropriate form of transmitter to use for this type of survey is one inwhich a neutrally buoyant horizontal electric dipole antenna 22 is towedbehind the deep-towed vehicle 18 at a height of a few meters to a fewtens of meters above the seafloor 6.

In the case of the receivers, there is an important advantage in using astatic recording device. It is necessary to measure the alternatingelectric or magnetic field at the seafloor 6, resulting from the signalemitted by the transmitter. In typical applications, the signal-to-noiseratio of the received signal will be critical to the success andresolution of the survey, and so must be maximised. Moving the receiverinevitably generates noise, whether the signal measured is magnetic orelectric field. In the case of electric fields, any motion of thereceiver through the conducting seawater medium 4 in the presence ofearth's geomagnetic field will generate an electromotive force acrossthe receiving antenna 24. Receiver movements will therefore map intospurious electric field signals in the recorded data. In the case ofmagnetic field recordings, there are also significant disadvantages tomoving the receiver. Most importantly, if vector rather than scalarmagnetometers are used (i. e. measuring individual directionalcomponents of the magnetic field), any variation in the orientation ofthe receiving antenna 24 will again lead to significant spurioussignals, since the magnetic detecting element will detect changes in thecomponent of the geomagnetic field aligned with it. As a consequence ofthese two effects, any translational movement of an electric fieldsensor or rotational movement of a magnetic field sensor will result incontamination of the received signal by motionally induced noise.

For these reasons, it is desirable to carry out a controlled sourceelectromagnetic survey to investigate or detect the presence of buriedhydrocarbons using a combination of a mobile horizontal electric dipolesource, equipped with a neutrally buoyant streamed antenna 22 andoperated just above the seafloor 6; and an array of electric and/ormagnetic field sensing receivers 24 placed statically on the seafloor 6as shown in FIG. 1. The receiving instruments 26 can be recovered usingacoustically actuated release mechanisms to separate them from theirballast weights, allowing them to return to the sea surface forrecovery, following standard oceanographic and marine geophysicalpractice.

The Effects of Propagation Through the Atmosphere

There is an important additional factor that is crucial to the successor otherwise of the technique described here. In shallow water depths,it is possible for signals from the transmitter to follow a propagationpath upwards through the water column to the surface; horizontallythrough the air; and back down through the water column to the seafloorreceiver. This ‘air wave’ component contains no information about thesub-seafloor resistivity. It tends to dominate the received signal inshallow water and at long source-to-receiver offsets. The effect of theair wave can be minimised by choosing appropriate transmissionfrequencies, and by targeting surveys on prospects in deep water and inwhich the target is at a relatively shallow depth below the seafloor.

FIGS. 2A, 2B and 2C show three cases of one-dimensional (1D) modellingshowing signal amplitude as a function of source to receiver range andfrequency.

Each of the figures corresponds to a different depth of seawateroverlying the seafloor.

FIG. 2A shows the modelled decay of electric field strength E as thedistance R between the source (transmitter) and detector (receiver)increases. The detector is separated from the receiver along a linewhich runs parallel to, and passes through the transmitting dipolesource. The model survey is undertaken above a semi-infinitesubterranean strata of uniform resistivity 1 Ωm, and in seawater ofdepth 750 m. The decay is shown for five different electromagneticfrequency components ranging from 0.5 Hz to 8 Hz.

FIG. 2B represents the results of a similar modelling to that shown inFIG. 2A, but for a seawater depth of 900 m.

FIG. 2C represents the corresponding results for a seawater depth of1300 m.

On all three sets of curves shown in FIGS. 2A, 2B and 2C, amplitudedecreases rapidly with range, as expected. Additionally, amplitudes at agiven range decrease with increasing frequency. This is because higherfrequencies have shorter skin depths, and so experience increasedattenuation. On each curve, the break in slope to a distinctly shallowergradient on the graph indicates the emergence of the air wave as thedominant signal at the receiver. For example, the curve corresponding toa signal component at 4 Hz shown in FIG. 2B shows that the air wavebecomes important in seawater of depth 900 m and using a 4 Hz sourcetransmission frequency when the detector is more than about 3.5 km awayfrom the source. It can be seen that this becomes an increasing problemin shallow water and at higher frequencies. For the technique describedhere to work most efficiently, the signal at the receiver must beprimarily due to propagation through the seafloor, and not due to theair wave. For instance, for this condition to be met at 5 to 6 kmsource-detector offset for the model subterranean strata describedabove, the frequency used for the survey should be no higher than 1 Hzin 1300 m water depth, or 0.5 Hz in 900 m water depth.

Modelling can thus be used to plan a survey in order to determine themaximum distance allowable between source and detectors for variousfrequencies. Suitable detector deployment positions and source frequencyranges can thus be determined in advance of performing a survey.

The Use of Survey Result Parameters Derived from Polarisation EllipseAnalysis

Use of a horizontal electric dipole antenna produces electromagneticfields at the seafloor that can be measured using electrometers orscalar or vector magnetometers. Scalar magnetometers are not used widelyin practice. Current practice is to measure two or more orthogonalcomponents of either electric or magnetic field. Up to six channels ofdata can usefully be recorded by the receiving instruments,corresponding to three orthogonal directional components each ofelectric and magnetic field. For operational reasons of instrumentcomplexity and data storage, in most cases a sub-set of these isrecorded.

The commonest arrangement currently is to use an orthogonal pair ofhorizontal electric dipole receiving elements in each receivinginstrument. In the horizontal plane, the transmitter generates bothradial and azimuthal components of electric field at the receiver, and,since these have different amplitudes and phases in general, theelectric field at the seafloor from a harmonic transmitter maps out apolarisation ellipse in the horizontal plane. The same applies in thecase of horizontal magnetic field; and if vertical components areincluded, the planar ellipse becomes a polarisation ellipsoid.

A straightforward approach to analysing controlled sourceelectromagnetic data is to resolve the observed fields into radial andazimuthal components. However this suffers from contamination of fieldvalues by errors arising from small inaccuracies in source and receiverorientation and position. As an alternative, we therefore propose thenovel approach of analysing the data from the recorded components interms of polarisation ellipse properties—e. g. the amplitude and/orphase of the component of the signal aligned along the major axis of theellipse. Since this value is much less dependent on the accuracy of thesource and receiver positioning—and in particular removes thesignificant errors that arise from even small uncertainties in receiverorientation—use of polarisation ellipse values can lead to smalleruncertainties in measured field properties at the receiver. We showbelow as an example that use of the amplitude along the major axis ofthe polarisation ellipse for horizontal electric field can be used as arobust measurement parameter for detecting the presence of buriedhydrocarbon layers. In principle, other polarisation ellipse parameterscan be used in a similar way. Parameters that could readily be measuredinclude amplitude or phase along the major axis; the horizontalpolarisation ellipse of either electric or magnetic field; or in eithercase a polarisation ellipsoid including a vertical component.

Presentation of Survey Results Using Normalised Values

As seen in FIGS. 2A, 2B and 2C above, a feature of a controlled sourceelectromagnetic survey over a sedimented seafloor is that the receivedfield amplitude drops very rapidly with increasing range. The receivedsignal properties also depend on the orientations of the source andreceiving dipoles. In a typical survey, the signal amplitude is likelyto vary by several orders of magnitude over the useful set ofsource-detector (transmitter-receiver) offsets; while the phase lag ofthe received signal also increases steadily with increasing offset. Theeffects of buried structure beneath the survey show up as deviations inthe signal from that which would be expected if the sub-surfaceconsisted only of a homogeneous, isotropic half space.

In order to visualise more clearly the effects of buried structure oncontrolled source electromagnetic signal properties, it is convenient tonormalise the observed signals with respect to some reference model. Asimple model to use would consist of a water layer of finite thicknessabove the survey of the true depth; and a homogeneous isotropic halfspace representing the seafloor below the survey. If appropriate, and ifthe relevant a priori information is available, a more complex referencemodel may be used, although it is desirable to use always the simplestreference model that can broadly represent the large scale backgroundproperties of the seafloor.

The normalisation is carried out for amplitude by dividing the observedfield by that calculated for the reference model, using the samesource-detector geometry as for the observed data. In the case of phase,the normalisation is carried out by subtracting the phase lag calculatedfor the reference model from that of the observed data. We show belowthat use of normalised values calculated in this way can dramaticallyenhance the visual presentation of controlled source electromagneticdata from a survey over a hydrocarbon reservoir. Our illustrations usenormalised amplitudes, but normalised phases could equally be used.

As an alternative, normalisation could be based on the survey dataitself, for example using data collected adjacent to the target.

The Varying Physical Response of a Survey Depending on SourceOrientation Geometry

Overall, the most crucial factor for the success or otherwise of thecontrolled source electromagnetic technique in practical applicationsrelated to hydrocarbon reservoirs is related to survey geometry.

FIG. 3 is a schematic plan view from above illustrating a suitableco-ordinate system for describing the relative placement of a controlledsource electromagnetic source 30 and an electromagnetic detector 32. Theposition of the detector 32 with respect to the source 30 is mostsuitably described in polar co-ordinates, with the centre of the source30 providing the origin of the polar co-ordinate system. The position ofthe detector 32 is defined by an azimuthal angle θ and a distance R. Thedetector 32 and source 30 are considered to be co-planar. The angle θ ismeasured clockwise from a line 33 passing through, and running parallelto, the source axis, as indicated in FIG. 3 by the line marked θ=0°. Adetector placed along the line 33, i. e. such that is has an azimuthalangle θ of 0°, is referred to as being in an end-on position. A detectorwith an azimuthal angle θ of 90°, such that it lies on the line 34marked θ=90° in FIG. 3, is referred to as being in a broadside position.The electric field strength at the detector is considered in terms of aradial component Eρ and an orthogonal azimuthal component E_(θ) asindicated in FIG. 3.

Depending on the relative placements and orientations of sources andreceivers, the physics of propagation of the signal through theseafloor—and so the net effect on the properties of the receivedsignal—can be more or less sensitive to different classes of subseafloor structure. As a simple illustration of this in the context ofhydrocarbon surveys, we consider a case in which the sub-seafloorstructure can be represented by a simple stack of horizontal layers (auseful first approximation for many situations in sedimentary basins).

FIG. 4A shows in schematic vertical section an exemplary subterraneanstrata configuration. A section of seafloor 42 lies beneath 800 m ofseawater 40. The strata beneath the seafloor 42 comprise a 1 km thickoverburden layer 44, representing sediments above a hydrocarbonreservoir layer 46. This overburden layer 44 has low resistivity,primarily due to aqueous saturation of pore spaces. The middle layer 46corresponds to a 100 m thick hydrocarbon reservoir and has a resistivityperhaps 100 times greater than the overburden 44. This is due to thepresence of the non-conducting hydrocarbon occupying much of the porespaces. Below the hydrocarbon reservoir layer 46 is a sub-reservoirregion 48 which has low resistivity due to its similarity to theoverburden layer 44 and extends downwards for an effectively infinitedistance.

In the case of an electromagnetic survey, we can consider the differingbehaviours of electric currents generated by the source resolved alonghorizontal and vertical directions. The relationships between theelectric currents flowing in two adjacent regions of space aredetermined by both galvanic (i. e. direct transfer of charge) andinductive effects. Since charge is conserved, current leaving one volumeof the sub-surface strata 44, 46, 48 and arriving in an adjacent volumealong the direction of current flow are related to each other by agalvanic mechanism. On the other hand, if two volumes are close togetherbut separated from each other along a direction orthogonal to currentflow, then the coupling between the currents flowing in the two volumeswill be primarily inductive.

Applying this to our model of a sub-seafloor structure shown in FIG. 4A,we can infer that the effect of the thin but resistive hydrocarbonreservoir layer 46 on the survey results will depend strongly on thedirection of flow of the currents generated by the source. If thecurrent at the base of the overburden layer 44 is dominantly horizontal,then coupling between this layer and the deeper layers 46, 48 will bedominated by inductive effects. Although little current will flow in thehydrocarbon reservoir layer 46, the inductive coupling between theoverburden 44 and the underlying layer 48 will be only mildly affected.Currents in the two conducting layers 44, 48 (overburden and underlyinglayers) will therefore be similar to the case without a hydrocarbonreservoir layer 46. The results of such a survey will therefore be onlyweakly affected by the presence of the hydrocarbon reservoir layer 46.

In contrast, if our survey generates significant vertical components ofelectric current flow in the overburden layer 44, then galvanic effectsalong the current path will be strongly affected by the thin resistivehydrocarbon reservoir layer 46. The resistive hydrocarbon reservoirlayer 46 will tend to block the current flow. As a result, currentsflowing in the deeper layer 48 will be much reduced; and the overallpattern of current flow in the overburden layer 44 will therefore besignificantly altered. In such a survey situation, the presence of thehydrocarbon will significantly influence the outcome.

Hence it becomes of paramount importance in designing a practical surveyapproach for detecting buried hydrocarbon reservoirs to distinguishbetween source (transmitter) and detector (receiver) geometries in whichcoupling between layers is largely inductive between current sheets in ahorizontal plane (in which case the survey has little sensitivity to thepresence of the reservoir); and those in which a significant componentof vertical current flow occurs, in which case blocking of the passageof this current flow by the reservoir leads to a survey which isstrongly sensitive to the presence of the reservoir.

A Specific Approach to Combining Two Contrasting Source Geometries WhenProspecting for Hydrocarbon Filled Reservoirs

By selecting a suitable survey geometry, it is possible to exploit thediverse properties of electromagnetic induction outlined above, bydesigning a survey in such a way that it provides data that aresensitive to the presence of thin resistive layers (exploiting largelygalvanic effects and vertical components of induced current flow); whilesimultaneously obtaining data that are more sensitive to the much largerscale structure, but relatively insensitive to a thin hydrocarbon layer(exploiting the dominantly inductive effects). The reason why the lattercomponent is essential is that many features other than hydrocarbonreservoirs can affect the resistivity beneath the seafloor and theresults of a survey. For example, resistivity often increases steadilywith depth in submarine sedimentary basins, due to the progressiveexpulsion of pore fluids by rising overburden pressure.

FIG. 4B shows in vertical section a highly schematic exemplarysubterranean strata configuration which exhibits increasing resistivitywith depth. A section of seafloor 52 lies beneath 800 m of seawater 50.The strata beneath the seafloor 52 comprise a series of layers ofsediment of increasing resistivity. A first layer 54 has a uniformresistivity of 1 Ωm and a thickness of 1 km. A second layer 56 has auniform resistivity of 5 Ωm and a thickness of 1 km. A third layer 58has a uniform resistivity of 10 Ωm and a thickness of 1 km. A fourthlayer 60 has a uniform resistivity of 50 Ωm and a thickness of 1 km.Beneath the fourth layer 60 is a fifth layer 62 which has a resistivityof 100 Ωm and extends downwards for an infinite extent.

The increasing resistivity indicated in FIG. 4B leads to longer skindepths, and hence to larger observed amplitudes and smaller observedphase lags for all detector placements. Although this does not exactlyreproduce the same galvanic effects of a thin resistive layer, theoverall effect on observed fields in the end-on geometry is likely to bevery similar to (and difficult or impossible to distinguish from) theeffect of a hydrocarbon reservoir. Designing a survey in such a way thatdifferent parts of the resulting data are more or less sensitive to thepresence of certain key features is essential for removing ambiguity inthe interpretation.

Recent studies of volcanic systems at mid-ocean ridges and of sedimentsbeneath resistive basalt layers (MacGregor et al., 1998, 2001; MacGregor& Sinha, 2000) have demonstrated that improved model resolution andreduced interpretational ambiguity can be achieved by using receivedelectric field components from two distinct source-detector geometries.This approach is now applied to the case of surveying for thinhydrocarbon reservoirs.

FIGS. 4C, 4D, 4E and 4F show survey simulation results for the twosubterranean strata configurations shown in FIGS. 4A and 4B. The methodrelies on collecting survey data using two distinct geometric modes. Thefirst mode corresponds to an end-on geometry, in which, as describedabove, the receiver lies along the axis of the transmitting dipole. Thesource-detector azimuth θ as defined in FIG. 3 is 0°, and the fieldobserved at the receiver is dominated by the radial component Eρ. Thesecond mode corresponds to a broadside geometry, in which, as describedabove, the source-detector azimuth θ is 90°, and the field observed atthe receiver is dominated by the azimuthal component E_(θ).

For the case of hydrocarbon exploration, where the target is a thinresistive layer embedded between a more conductive underburden andoverburden, such as that shown schematically in FIG. 4A, the receivedsignal depends on the azimuthal angle θ of the detector. Because of thetransmitter field pattern, the end-on, or Eρ, geometry results in asignificant contribution to the observed field at the seafloor by thevertical component of current flow. The broadside, or E_(θ), geometryresults in fields at the seafloor that are more dependent on thecontribution of inductively coupled currents flowing in horizontalplanes. As a result, the presence of a resistive hydrocarbon reservoirlayer produces a significant increase at certain ranges in the end-on(Eρ) fields, while having virtually no effect on the broadside (E_(θ))fields. The result is ‘splitting’ of amplitudes between the two modes,and this splitting is characteristic of a structure in which resistivityfirst increases, in this case due to a hydrocarbon layer, and thendecreases with depth.

FIG. 4C is a graph showing the modelled amplitudes of electric field Ealong the major axes of the horizontal polarisation ellipses for twosource-detector geometries and as a function of distance R betweensource and detector. These are determined for the model of sub-seafloorstructure indicated in FIG. 4A. Four curves are shown. The curvelabelled E₀ shows the field strength as a function of distance forend-on geometries, the curve labelled Ego shows the same for broadsidegeometries. The curves labelled N₀ and N₉₀ are the correspondingnormalisation curves. These are determined for the same geometries asfor E₀ and E₉₀, but represent the modelled amplitudes of electric fieldalong the major axes of the horizontal polarisation ellipses where thesubterranean strata comprise a homogeneous isotropic half space ofresistivity 1 Ωm, such as previously described.

FIG. 4D is a graph showing the set of curves which correspond to thosecurves shown in FIG. 4C, but for the subterranean strata configurationshown in FIG. 4B.

FIG. 4E is a graph showing the normalised values of the fields E₀ andE₉₀ shown in FIG. 4C. These fields have been normalised by the fields N₀and N₉₀, also shown in FIG. 4C, as discussed above.

FIG. 4F is a graph showing the normalised values of the fields E₀ andE₉₀ shown in FIG. 4D. These fields have been normalised by the fields N₀and N₉₀, also shown in FIG. 4D, as discussed above.

It can be seen in FIG. 4E that the presence of the reservoir hasrelatively little effect on the broadside amplitudes (i. e. the curvelabelled E₉₀/N₉₀ is close to unity), but that between 4 and 6 km offset,it produces a substantial increase in the end-on amplitudes (i. e. thecurve labelled E₀/N₀ is strongly enhanced over this range). Thehydrocarbon layer in this particular model leads to amplitude splittingbetween the two geometrical modes by about a factor of 10.

It can be seen in FIG. 4F that the steadily increasing resistivity withdepth model shown in FIG. 4B strongly affects both geometries (i. e.both curves are strongly enhanced between 4 and 6 km offset) and nosignificant splitting between the two geometric modes occurs.

The end-on geometry data alone cannot distinguish between the twosubterranean strata configurations shown in FIGS. 4A and 4B. Thesecomparative examples conclusively demonstrate the fact that collectionof survey data from both end-on and broadside geometries is needed, inorder to reliably detect the hydrocarbon layer and eliminate“false-positive” detection of a rock formation containing no hydrocarbonreservoir, but merely having an increasing resistivity with depth, whichis not unusual.

Lastly, the normalised curves in FIGS. 4E and 4F start off close tounity, at short ranges where the data are insensitive to the buriedstructure. The effects of the buried structure become greatest atbetween 5 and 6 km range. Beyond this, the normalised curves rapidlyreturn towards a value of 1, because at long ranges, air wavepropagation (insensitive to sub-seafloor structure) begins to dominate,thus masking the effects of the sub surface. As a result, the presenceof the target structure can best be observed in controlled sourceelectromagnetic data of this type over a relatively narrow window ofsource-detector separation ranges.

An efficient electromagnetic survey may use a large number of detectorsdistributed over a target area and a signal transmitter which operatescontinuously as it is towed along an extended tow path. This followsfrom the relatively low cost of deploying detectors and the relativelyhigh cost of deploying a transmitter.

FIG. 5 is a schematic plan view showing an example layout of sixteendetectors 64 distributed across a section of seafloor 65 abovesubterranean reservoir 66. The reservoir 66 has a linear extent ofseveral km and its boundary is indicated by a heavy line 67. In thisexample, the detectors 64 are uniformly distributed in a square-gridpattern so as to approximately cover the substantially square reservoir66.

In performing a survey, a source (not shown) starts from the locationmarked ‘A ’ in FIG. 5 and is towed, whilst broadcasting continuously,along a path indicated by the broken line 68, the survey is completedwhen the source reaches the location marked ‘B’. Data are continuouslyrecorded by the detectors 64 throughout the towing process and theposition of the source transmitter relative to the detector network isalso logged.

During the towing process, each of the detectors 64 presents severaldifferent orientations to the source. For example, when the source isdirectly above the detector position D1 and on the vertical section ofthe tow path, the detectors at positions D2 and D3 are at differentranges in an end-on position, the detectors at positions D4 and D5 areat different ranges in a broadside position and the detector at positionD6 is midway between. However, when the source later passes over thedetector position D1 when on the horizontal section of the tow path, thedetectors at positions D2 and D3 are now in a broadside position, andthe detectors at position D4 and D5 are in an end-on position. Thus, inthe course of a survey, and in conjunction with the positionalinformation of the source, data from the detectors 64 can be used toprovide details of the signal transmission through the subterraneanstrata for a comprehensive range of distances and orientations betweensource and detector, each with varying galvanic and inductivecontributions to the signal propagation. In this way a simple continuoustowing of the source transmitter can provide a detailed survey whichcovers the extent of the subterranean reservoir 66.

Although the above example is based on a square detector grid, it willbe understood that a wide variety of detector placements may be used,for example other high symmetry regular grids, such as triangular orrectangular, may be used. In addition irregular grids may be used thathave no high level of symmetry.

On the Detection of the Edges of a Reservoir by this Technique

We showed above that our proposed arrangement of survey geometries canboth detect a hydrocarbon reservoir, and distinguish it from a generalincrease in resistivity with depth. The modelling though was based onlyon 1-D assumptions, i. e. horizontal layers of infinite horizontalextent. By using higher dimensional modelling, we show below that thesurvey technique also works well in the case of reservoirs of finiteextent; and indeed that it can be used as a reliable means of locatingthe edges of a reservoir structure.

FIG. 6A shows a schematic section view of a model subterranean strataconfiguration containing a hydrocarbon reservoir layer of finitehorizontal extent. A section of seafloor 70 lies beneath a body of sea72 which is 1.15 km deep and has a resistivity of 0.31 Ωm. A planarhydrocarbon reservoir 74 is 0.15 km thick and its base is 1 km below theseafloor 70. The reservoir 74 has a resistivity of 100 Ωm and extendsinfinitely out of the plane of FIG. 6A, and semi-infinitely within theplane of FIG. 6A. A uniform sedimentary structure 76 of resistivity 1 Ωmsurrounds the reservoir 74 and fills the remaining space beneath theseafloor 70. For the purposes of the model, two transmitter positionsare considered. A first dipole transmitter 75 is aligned parallel to anddirectly above the edge 77 of the reservoir 74 on the seafloor 70. Thedipole axis of the transmitter 75 is perpendicular to the plane of FIG.6A and a cross is used to simply indicate its position rather thanrepresent its structure. A second transmitter 78 is also on the seafloor70 and aligned parallel to the first transmitter 75. The secondtransmitter 78 is horizontally displaced from the first transmitter 75by 1 km in a direction which positions it directly above the reservoir74 as indicated in FIG. 6A.

FIGS. 6B and 6C show the results of 2.5D modelling (3-D source, 2-Dresistivity structure) of the exemplary subterranean strataconfiguration shown in FIG. 6A for the first and second transmitterpositions 75 and 78 respectively in order to investigate the effects ofthe finite areal extent of the resistive hydrocarbon reservoir 74. FIGS.6B and 6C are plan views representing the normalised fields at theseafloor 70 as a function of position. The normalised field componentshown in each case is the semi-major axis of the horizontal polarisationellipse of electric field.

In FIG. 6B the first transmitter 75 is at the origin of the localco-ordinate system and its dipole axis is parallel to the vertical axisin this system. In FIG. 6C the second transmitter 78 is at the origin ofthe local co-ordinate system and its dipole axis is parallel to thevertical axis in this system.

The normalised field amplitude is shown in a grey scale representation,while contour lines indicate the absolute value of amplitude. FIGS. 6Band 6C show that the reservoir structure causes substantial increases inthe fields at positions close to end-on geometries, this is especiallyso when the source and detector are both over the reservoir, as in FIG.6C. If the source or detector are at or outside the edge of thereservoir, the amplitude increase is not seen. At detector locationsclose to broadside geometry, the increase in amplitude is again notseen, even if both source and detector are over the reservoir. FIGS. 6Band 6C illustrate that the amplitude splitting effect is observable overreservoirs of finite size, as well as over the 1-D (layered) structuresdiscussed previously.

The marked difference between FIGS. 6B and 6C shows that the observedfields are extremely sensitive to the location of the source(transmitter) with respect to the edge of the reservoir. In FIG. 6B, themarked asymmetry of the fields about the y axis shows that-provided thesource is over the reservoir-the observed fields are also highlysensitive to the location of the detector (receiver) with respect to theedge of the reservoir. Thus by careful siting of both source tow tracksand detectors relative to a suspected hydrocarbon bearing structure, thesurvey method described here can provide detailed information on theareal extent of the hydrocarbon, as well as on its existence orotherwise.

In addition to initially surveying for subterranean hydrocarbonreservoirs, the utility of the invention in detecting the edges ofreservoirs makes it applicable to assessing changes in hydrocarbonreservoir content over a period of time. This is of particular value,for example, where a hydrocarbon reservoir is being exploited. As waterreplaces the hydrocarbon which is drawn from the reservoir, regularfollow up surveys can be used to determine the change in thedistribution of the remaining hydrocarbon. Such surveys could thereforebe used to characterise the evolution of the reservoir through time, andhence enable more efficient extraction and reservoir management.

Source and Receiver Locations-design Considerations for Real Surveys

The methodology described in this document can be readily applied toreal surveys for hydrocarbon resources on continental margins. Therequired transmitter and receiver characteristics in terms of power,signal to noise ratios and operating parameters can be met by existinginstrumental technology.

In order to plan a successful survey, the following factors areimportant:

An understanding of the need to collect observations over an appropriateset of source-detector separation ranges, and using both broadside andend-on surveying geometries;

A designed layout of the detector array and of source tow line locationsand orientations optimised for the above;

A designed layout for the survey that also takes account of the need tolocate the limits of the areal extent of any reservoir, based on theapproach presented in FIGS. 6A, 6B and 6C;

Prior modelling to establish the range of transmission frequencies, andthe set of source-detector separation ranges, that should be used,taking into consideration the expected resistivity structure of theseafloor, the depth of the target, the depth of the seafloor, and theinfluence of the air wave on the data collected.

Conclusions

We have shown that the application of a specific set of methods to acontrolled source electromagnetic survey of the seafloor can allow thetechnique to be successfully applied to the problem of detectinghydrocarbons beneath the seafloor.

Specifically, we have shown that: The survey should be carried out usinga combination of a mobile horizontal electric dipole source transmitter,equipped with a neutrally buoyant streamed antenna and operated justabove the seafloor; and an array of electric and/or magnetic fieldsensing receivers placed statically on the seafloor.

The effects of propagation of the signal through the atmosphere (the‘air wave’) are significant at high frequencies, at long source-detectorseparation distances, and in shallow water. This effect limits theapplicability of the method by favouring detection of structuresrelatively shallowly situated beneath the seafloor, but in deep water;and limits the choice of frequencies and source-detector offsets for thesurvey.

Improvements in data analysis can be achieved by making use of fieldmeasurement parameters aligned along the major axis of the polarisationellipse at the seafloor.

For purposes of data presentation and interpretation it is desirable tomake use of field parameters that have been normalised with reference toan appropriate simplified model of the sub-seafloor structure.

In order to resolve the presence of any hydrocarbon, and to distinguishits effect on the data from other likely structures and reduce theambiguity of interpretation, it is essential to collect survey data fromboth the end-on and broadside geometric configurations; and to analysethe data in terms of splitting between the two geometric modes.

2.5 D modelling shows that provided an appropriately designed array ofreceivers, and an appropriate set of transmitter tow tracks, are used,then the method can yield valuable information about the limits of theareal extent of any sub-seafloor hydrocarbon reservoir, as well asdetecting its presence.

Finally it will be understood that the invention is equally applicableto surveying of freshwater, for example large lakes, so that referencesto seafloor, seawater etc. should not be regarded as limiting.

REFERENCES

-   1. Sinha, M. C., Patel, P. D., Unsworth, M. J., Owen, T. R. E. &    MacCormack, M. R. G. An active source electromagnetic sounding    system for marine use. Mar. Geophys. Res., 12, 1990, 59-68.-   2. MacGregor, L. M., Constable, S. C. & Sinha, M. C. The RAMESSES    experiment III: Controlled source electromagnetic sounding of the    Reykjanes Ridge at 57° 45′ N. Geophysical Journal International,    135, 1998, 773-789.-   3. MacGregor, L. M. & Sinha, M. C. Use of marine controlled source    electromagnetic sounding for sub-basalt exploration. Geophysical    Prospecting, 48, 2000, 1091-1106.-   4. MacGregor, L., Sinha, M. & Constable, S. Electrical resistivity    structure of the Valu Fa Ridge, Lau Basin, from marine controlled    source electromagnetic sounding. Geophys. J. Int., 146, 217-236,    2001.-   5. Evans, R. L., Sinha, M. C., Constable, S. C. & Unsworth, M. J. On    the electrical nature of the axial melt zone at 13′ N on the East    Pacific Rise. J Geophys. Res., 99, 1994, 577-588.-   6. WO 00/13046-   7. WO 01/57555

1. An electromagnetic survey method for surveying an area previouslyidentified as potentially containing a subsea hydrocarbon reservoir,comprising: providing an electromagnetic source having a dipole axis andfirst and second detectors; moving the electromagnetic source with thedipole axis of the electromagnetic source aligned end-on relative to thefirst detector so that a first survey data set is sensitive to resistivehydrocarbon layers exploiting largely galvanic effects; moving theelectromagnetic source with the dipole axis of the electromagneticsource aligned with the dipole axis of the electromagnetic sourcealigned broadside relative to the second detector so that a secondsurvey data set is relatively insensitive to resistive hydrocarbonlayers exploiting dominantly inductive effects; moving theelectromagnetic source relative to each detector to collect data over arange of source-to-detector distances; and processing the first surveydata set and the second survey data set.
 2. An electromagnetic surveymethod according to claim 1, comprising obtaining the first and secondsurvey data sets concurrently.
 3. An electromagnetic survey methodaccording to claim 1, wherein a right angle is formed between a firstline leading from the first detector to the electromagnetic source and asecond line leading from the second detector to the electromagneticsource.
 4. An electromagnetic survey method according to claim 1,further comprising selecting the electromagnetic source to be a mobilehorizontal electric dipole source equipped with a streamed antenna whichis towed above the seafloor to obtain the first and second survey datasets.
 5. An electromagnetic survey method according to claim 1, whereineach detector is static while the first and second survey data sets areobtained.
 6. An electromagnetic survey method according to claim 5,further comprising placing each detector on the seafloor while the firstand second survey data sets are obtained.